1. Field of the Invention
This invention relates to non-invasive means of determining cardiac output or pulmonary capillary blood flow in patients and, more specifically, to partial re-breathing systems and methods for determining cardiac output or pulmonary capillary blood flow in patients.
2. Statement of the Art
It is important in many medical procedures to determine or monitor the cardiac output or the pulmonary capillary blood flow of a patient. Cardiac output as the volume of blood pumped by the heart over a given period of time. Pulmonary capillary blood flow is the volume of blood that participates in gas exchange in the lungs. Techniques are known and used in the art which employ the use of catheters inserted into blood vessels at certain points (e.g., into the femoral artery, the jugular vein, etc.) to monitor blood temperature and pressure and to thereby determine the cardiac output or pulmonary capillary blood flow of the patient. Although such techniques can produce a reasonably accurate result, the invasive nature of these procedures has a high potential for causing morbidity or mortality.
Adolph Fick""s formula for calculating cardiac output, which was first proposed in 1870, has served as the standard by which other means of determining cardiac output and pulmonary capillary blood flow have since been evaluated. Fick""s well-known equation, which is also referred to as the Fick Equation, written for carbon dioxide (CO2), is:       Q    =                  V                  CO          2                            (                              C                          v                              CO                2                                              -                      C                          a                              CO                2                                                    )              ,
where Q is cardiac output, VCO2 is the amount of CO2 excreted by the lungs, or xe2x80x9cCO2 elimination,xe2x80x9d and CaCO2 and CvCO2 are the CO2 contents of arterial blood and venous blood, respectively. Notably, the Fick Equation presumes an invasive method (i.e., catheterization) of calculating cardiac output or pulmonary capillary blood flow because the arterial blood and mixed venous blood must be sampled in order to directly determine the CO2 contents of arterial blood and venous blood.
It has been shown, however, that by using the principles embodied in the Fick Equation, non-invasive means may be employed to determine cardiac output or pulmonary capillary blood flow. That is, expired CO2 levels, measured in terms of fraction of expired gases that comprise CO2 (fCO2) or in terms of partial pressure of CO2 (PCO2), can be monitored and employed to estimate the content of CO2 in the arterial blood. Thus, a varied form of the Fick Equation may be employed to estimate cardiac output or pulmonary capillary blood flow based on observed changes in fCO2 or PCO2.
An exemplary use of the Fick Equation to non-invasively determine cardiac output or pulmonary capillary blood flow includes comparing a xe2x80x9cstandardxe2x80x9d ventilation event to a change in expired CO2 values and a change in excreted volume of CO2, which is referred to as carbon dioxide elimination or CO2 elimination (VCO2), which may be caused by a sudden change in ventilation. Conventionally, a sudden change in effective ventilation has been caused by having a patient inhale or breathe a volume of previously exhaled air. This technique is typically referred to as xe2x80x9cre-breathing.xe2x80x9d
Some re-breathing techniques have used the partial pressure of end-tidal CO2 (PetCO2 or etCO2) to approximate the content of CO2 in the arterial blood of a patient while the patient""s lungs act as a tonometer to facilitate the measurement of the CO2 content of the venous blood of the patient.
By further modification of the Fick Equation, it may be assumed that the CO2 content of the patient""s venous blood does not change within the time period of the perturbation. Thus, the need to directly calculate the CO2 content of venous blood was eliminated by employing the so-called xe2x80x9cpartial re-breathingxe2x80x9d method. (See, Capek et al., xe2x80x9cNoninvasive Measurement of Cardiac Output Using Partial CO2 Rebreathingxe2x80x9d, IEEE Transactions on 
The carbon dioxide elimination of the patient may be non-invasively measured as the difference per breath between the volume of carbon dioxide inhaled during inspiration and the volume of carbon dioxide exhaled during expiration, and is typically calculated as the integral of the carbon dioxide signal times the rate of flow over an entire breath. The volume of carbon dioxide inhaled and exhaled may each be corrected for any deadspace or for any intrapulmonary shunt.
The partial pressure of end tidal carbon dioxide is also measured in re-breathing processes. The partial pressure of end-tidal carbon dioxide, after correcting for any deadspace, is typically assumed to be approximately equal to the partial pressure of carbon dioxide in the alveoli (PACO2) of the patient or, if there is no intrapulmonary shunt, the partial pressure of carbon dioxide in the arterial blood of the patient (PaCO2). Conventionally employed Fick methods of determining cardiac output or pulmonary capillary blood flow typically include a direct, invasive determination of CvCO2 by analyzing a sample of the patient""s mixed venous blood. The re-breathing process is typically employed to either estimate the carbon dioxide content of mixed venous blood (in total re-breathing) or to obviate the need to know the carbon dioxide content of the mixed venous blood (by partial re-breathing) or determine the partial pressure of carbon dioxide in the patient""s venous blood (PvCO2).
Re-breathing processes typically include the inhalation of a gas mixture that includes carbon dioxide. During re-breathing, the carbon dioxide elimination of a patient typically decreases. In total re-breathing, carbon dioxide elimination decreases to near zero. In partial re-breathing, carbon dioxide elimination does not cease. Thus, in partial re-breathing, the decrease in carbon dioxide elimination is not as large as that of total re-breathing.
Re-breathing can be conducted with a re-breathing circuit, which causes a patient to inhale a gas mixture that includes carbon dioxide. FIG. 1 schematically illustrates a conventional ventilation system that is typically used with patients who require assisted breathing during an illness, during a surgical procedure, or during recovery from a surgical procedure. The conventional ventilator system 10 includes a tubular portion 12 that may be inserted into the trachea of a patient by known intubation procedures. The end 14 (i.e., the end most distant from the patient) of the tubular portion 12 may be fitted with a Y-piece 16 that interconnects an inspiratory hose 18 and an expiratory hose 20. Both the inspiratory hose 18 and expiratory hose 20 may be connected to a ventilator machine (not shown), which delivers air into the breathing circuit through the inspiratory hose 18. A one-way valve 22 is positioned on the inspiratory hose 18 to prevent exhaled gas from entering the inspiratory hose 18 beyond the valve 22. A similar one-way valve 24 on the expiratory hose 20 limits movement of inspiratory gas into the expiratory hose 20. Exhaled air flows passively into the expiratory hose 20.
With reference to FIG. 2, an exemplary known re-breathing ventilation circuit 30 is shown. Re-breathing circuit 30 includes a tubular portion 32 insertable into the trachea of a patient by known intubation procedures. Gases may be provided to the patient from a ventilator machine (not shown) via an inspiratory hose 34 interconnected with tubular portion 32 by a Y-piece 36. Tubular portion 32 and an expiratory hose 38 are also interconnected by Y-piece 36. An additional length of hose 40 is provided in flow communication with the tubular portion 32, between the tubular portion 32 and the Y-piece 36, and acts as a deadspace for receiving exhaled gas. A three-way valve 42, generally positioned between the Y-piece 36 and the opening to the additional length of hose 40, is constructed for intermittent actuation to .selectively direct the flow of gas into or from the additional length of hose 40. That is, at one setting, the valve 42 allows inspiratory gas to enter the tubular portion 32 while preventing movement of the gas into the additional length of hose 40. At a second setting, the valve 42 allows exhaled gas to enter into the expiratory hose 38 while preventing movement of gas into the additional length of hose 40. At a third setting, the three-way valve 42 directs exhaled air to enter into the additional length of hose 40 and causes the patient to re-breathe the exhaled air on the following breath thereby, effect re-breathing and to cause a change in the effective ventilation of the patient.
The change in CO2 elimination and in the partial pressure of end-tidal CO2 caused by the change in ventilation in the system of FIG. 2 can then be used to calculate the cardiac output or pulmonary capillary blood flow of the patient. Sensing and/or monitoring devices may be attached to the re-breathing ventilation circuit 30 between the additional length of hose 40 and the tubular portion 32. The sensing and/or monitoring devices may include, for example, means 44 for detecting CO2 concentration and means 46 for detecting respiratory flow parameters during inhalation and exhalation. These sensing and/or monitoring devices are typically associated with data recording and display equipment (not shown). One problem encountered in use of the conventional re-breathing system is that the volume of the deadspace provided by the additional length of hose 40 is fixed and may not be adjusted. As a result, the amount of deadspace provided in the circuit for a small adult to effect re-breathing is the same amount of deadspace available for a large adult to effect re-breathing, and the resulting changes in CO2 values for patients of different sizes or breathing capacities, derived from fixed-deadspace systems, can produce inadequate evaluation of a patient""s cardiac output or pulmonary capillary blood flow. Further, the three-way valve 42 of the system is, expensive and significantly increases the cost of the ventilation device.
During total re-breathing, the partial pressure of end-tidal carbon dioxide (PetCO2) is typically assumed to be equal to the partial pressure of carbon dioxide in the venous blood (PvCO2) of the patient, as well as to the partial pressure of carbon dioxide in the arterial blood (PaCO2) of the patient and to the partial pressure of carbon dioxide in the alveolar blood (PACO2) of the patient. The partial pressure of carbon dioxide in blood may be converted to the content of carbon dioxide in blood by means of a carbon dioxide dissociation curve.
In partial re-breathing, measurements during normal breathing and subsequent re-breathing are substituted into the carbon dioxide Fick equation. This results in a system of two equations and two unknowns (carbon dioxide content in the mixed venous blood and cardiac output), from which cardiac output or pulmonary capillary blood flow can be determined without knowing the carbon dioxide content of the mixed venous blood (CvCO2).
Total re-breathing is a somewhat undesirable means of measuring cardiac output or pulmonary capillary blood flow because the patient is required to breathe directly into and from a closed volume of gases (e.g., a bag) in order to produce the necessary effect. Moreover, it is typically impossible or very difficult for sedated or unconscious patients to actively participate in inhaling and exhaling into a fixed volume.
Known partial re-breathing methods are also advantageous over invasive techniques of measuring cardiac output or pulmonary capillary blood flow because partial re-breathing techniques are non-invasive, use the accepted Fick principle of calculation, are easily automated, and facilitate the calculation of cardiac output or pulmonary capillary blood flow from commonly monitored clinical signals. However, known partial re-breathing methods are somewhat undesirable because they are a less accurate means of measuring the cardiac output or pulmonary capillary blood flow of non-intubated or spontaneously breathing patients, may only be conducted intermittently (usually at intervals of at least about four minutes), and result in an observed slight, but generally clinically insignificant, increase in arterial CO2 levels. Moreover, the apparatus typically employed in partial re-breathing techniques do not compensate for differences in patient size or breathing capacities. In addition, many devices employ expensive elements, such as three-way valves, which render the devices too expensive to be used as disposable units.
Thus, there is a need for adjustable deadspace re-breathing apparatus that compensate for differences in the sizes or breathing capacities of different patients, that may be employed to provide a more accurate and continuous measurement of gases exhaled or inhaled by a patient, and are less expensive than conventional re-breathing apparatus and, thereby, facilitate use of the adjustable deadspace re-breathing apparatus as a single-use, or disposable, product. There is also a need for a more accurate method of estimating the cardiac output or pulmonary capillary blood flow of a patient.
In accordance with the present invention, apparatus and methods for measuring the cardiac output or pulmonary capillary blood flow of a patient are provided. The apparatus of the present invention includes a deadspace (i.e., volume of re-breathed gases), the volume of which can be adjusted without changing airway pressure. The invention also includes methods of adjusting the volume of deadspace to obtain a more accurate cardiac output or pulmonary capillary blood flow value. A modified form of the Fick Equation may be employed with the adjustable deadspace volume to calculate the cardiac output or pulmonary capillary blood flow of the patient. The apparatus of the present invention also employs significantly less expensive elements of construction, thereby facilitating the use of the apparatus as a disposable product.
The apparatus and methods of the present invention apply a modified Fick Equation to calculate changes in partial pressure of carbon dioxide (PCO2), flow, and concentration to evaluate the cardiac output or pulmonary capillary blood flow of a patient. The traditional Fick Equation, written for CO2 is:       Q    =                  V                  CO          2                            (                              C                          v                              CO                2                                              -                      C                          a                              CO                2                                                    )              ,
where Q is pulmonary capillary blood flow (xe2x80x9cPCBFxe2x80x9d), VCO2 is the output of CO2 from the lungs, or xe2x80x9cCO2 eliminationxe2x80x9d, and CaCO2 and CvCO2 are the CO2 contents of the arterial blood and venous blood CO2, respectively. It has been shown in the prior work of others that cardiac output can be estimated from calculating the change in the fraction or volume of CO2 exhaled by a patient and the partial pressure of end-tidal CO2 as a result of a sudden change in ventilation. That can be done by applying a differential form of the Fick Equation, as follows:       Q    =                            V                      CO                          2              1                                                (                                    C                              v                1                                      -                          C                              a                1                                              )                    =                        V                      CO                          2              2                                                (                                    C                              v                2                                      -                          C                              a                2                                              )                      ,
where CaCO2 is the CO2 content of the arterial blood of a patient, CvCO2 is the CO2 content of the venous blood of the patient, and the subscripts 1 and 2 refer to measured values before a change In ventilation and measured values during a change in ventilation, respectively. The differential form of the Fick Equation can, therefore, be rewritten as:       Q    =                                                      V                              CO                                  2                  1                                                      -                          V                              CO                                  2                  2                                                                                        (                                                C                                      v                    1                                                  -                                  C                                      a                    1                                                              )                        -                          (                                                C                                      v                    2                                                  -                                  C                                      a                    2                                                              )                                      ⁢                  xe2x80x83                ⁢        or        ⁢                  xe2x80x83                ⁢        Q            =                                    Δ            ⁢                          xe2x80x83                        ⁢                          V                              CO                2                                                          Δ            ⁢                          xe2x80x83                        ⁢                          C                              a                                  CO                  2                                                                    =                              Δ            ⁢                          xe2x80x83                        ⁢                          V                              CO                2                                                          s            ⁢                          xe2x80x83                        ⁢            Δ            ⁢                          xe2x80x83                        ⁢                                          Pet                ⁢                CO                            2                                            ,
where xcex94VCO2 is the change in CO2 elimination in response to the change in ventilation, xcex94CaCO2 is the change in the CO2 content of the arterial blood of the patient in response to the change in ventilation, xcex94PetCO2 is the change in the partial pressure of end-tidal CO2, and s is the slope of a CO2 dissociation curve known in the art. The foregoing differential equation assumes that there is no appreciable change in venous CO2 concentration during the re-breathing episode, as demonstrated by Capek. Also, a CO2 dissociation curve, well known in the art, is used for determining CO2 concentration based on partial pressure measurements.
In previous partial re-breathing methods, a deadspace, which may comprise an additional 50-250 ml capacity of air passage, was provided in the ventilation circuit to decrease the effective alveolar ventilation. In the present invention, a ventilation apparatus is provided with a deadspace having an adjustable volume to provide a change in ventilation for determining accurate changes in CO2 elimination and in partial pressure of end-tidal CO2 that is commensurate with the requirements of patients of different sizes or breathing capacities. In one embodiment of the ventilation apparatus, selectively adjustable deadspace is provided into which the patient may exhale and from which the patient may inhale. Thus, the adjustable deadspace volume of the apparatus accommodates a variety of patient sizes or breathing capacities (e.g., from a small adult to a large adult). As a result, the patient is provided with a volume of re-breathable gas commensurate with the patient""s size or breathing capacity, which decreases the effective ventilation of the patient without changing the airway pressure of the patient. Because airway and intra-thoracic pressure are not affected by the re-breathing method of the present invention, cardiac output and pulmonary capillary blood flow are not significantly affected by re-breathing.
In an alternative method, the volume of deadspace may be effectively lessened by selectively leaking exhaled gas from the ventilation system to atmosphere or to a closed receptacle means during inspiration. Similarly, additional carbon dioxide may be introduced into the deadspace to increase the effective deadspace volume. Changing the effective deadspace volume in such a manner has substantially the same effect as changing the actual volume of the deadspace of the ventilation apparatus.
The ventilation apparatus of the present invention includes a tubular portion, which is also referred to as a conduit, to be placed in flow communication with the airway of a patient. The conduit of the ventilation apparatus may also be placed in flow communication with or include an inhalation course and an exhalation course, each of which may include tubular members or conduits. In a common configuration, the inhalation course and exhalation course may be interconnected in flow communication between a ventilator unit (i.e., a source of deliverable gas mechanically operated to assist the patient in breathing) and the patient. Alternatively, however, a ventilator unit need not be used with the ventilation apparatus. For example, inhaled air and exhaled air may be taken from or vented to atmosphere. Other conventional equipment commonly used with ventilator units or used in ventilation of a patient, such as a breathing mask, may be used with the inventive ventilation apparatus.
A pneumotachometer for measuring gas flow and a capnometer for measuring CO2 partial pressure are provided along the flow path of the ventilation apparatus and, preferably, in proximity to the conduit, between the inhalation and exhalation portions of the ventilation apparatus and the patient""s lungs. The pneumotachometer and capnometer detect changes in gas concentrations and flow and are preferably in electrical communication with a computer programmed (i.e., by software or embedded hardware) to store and evaluate, in substantially real time, the measurements taken by the detection apparatus. Other forms of detection apparatus may, alternatively or in combination with the pneumotachometer and the capnometer, be employed with the ventilation apparatus of the present invention.
Deadspace having an adjustable volume is provided in flow communication with the conduit. In particular, the deadspace is in flow communication with the exhalation portion of the ventilation apparatus (e.g., the expiratory course), and may be in flow communication with the inhalation portion (e.g., the inspiratory course) of the ventilation apparatus. In one embodiment, the volume of the deadspace may be manually adjusted. Alternatively, electromechanical means may be operatively associated with the computer and with the deadspace to provide automatic adjustment of the volume of the deadspace in response to the patient""s size or breathing capacity or in response to changes in the ventilation or respiration of the patient.
In an alternative embodiment, a tracheal gas insufflation (xe2x80x9cTGIxe2x80x9d) apparatus is employed to provide the change in ventilation necessary to determine pulmonary CO2 changes and to determine the cardiac output or pulmonary capillary blood flow of a patient in accordance with the differential Fick Equation disclosed previously. Tracheal gas insufflation apparatus are known, and are typically used to flush the deadspace of the alveoli of the lungs and to replace the deadspace with fresh gas infused through the TGI apparatus. That is, fresh gas is introduced to the central airway of a patient to improve alveolar ventilation and/or to minimize ventilatory pressure requirements. A TGI apparatus may be interconnected, for example, by means of a catheter, with a ventilator apparatus and includes a means of introducing fresh gas into the breathing tube and into the lungs of the patient. The TGI apparatus may be used in the methods of the present invention to determine baseline measurements of CO2 elimination, partial pressure of end tidal CO2, or partial pressure of alveolar CO2 during TGI. When the TGI system is turned off, a deadspace is formed by the patient""s trachea and the endo-tracheal tube of the TGI apparatus, which facilitates measurement of a change in the partial pressure of CO2 and in the amount of CO2 eliminated by the patient that may be evaluated in accordance with the method of the present invention. Further, the catheter of the TGI apparatus may be variably positioned within the trachea of the patient to further adjust the deadspace volume.
During re-breathing, the deadspace provided by the apparatus of the present invention facilitates a rapid drop in CO2 elimination, which thereafter increases slightly and slowly as the functional residual lung gas capacity, which is also referred to as functional residual capacity or xe2x80x9cFRCxe2x80x9d, equilibrates with the increase in the partial pressure of CO2 in the alveoli. Partial pressure of end tidal CO2 increases at a slower rate than CO2 elimination following the addition of deadspace, depending on alveolar deadspace and the cardiac output or pulmonary capillary blood flow of the patient, but then stabilizes to a new level. A xe2x80x9cstandard,xe2x80x9d or baseline, breathing episode is conducted for a selected period of time immediately preceding the introduction of a deadspace into the breathing circuit (i.e., immediately preceding re-breathing) and CO2 elimination and partial pressure of end tidal CO2 values are determined based on measurements made during the xe2x80x9cstandardxe2x80x9d breathing event. These values are substituted as the values VCO2 and CaCO2 in the differential Fick Equation. Carbon dioxide elimination and partial pressure of end tidal CO2 values are also determined from measurements taken for a predetermined amount of time (e.g., approximately thirty seconds) following the introduction of a deadspace (i.e., after the onset of re-breathing) during partial re-breathing to provide the second set of values (subscript 2 values) in the differential Fick Equation. Thus, the predetermined amount of time at which the second set of values are obtained may be about the same as the duration of partial re-breathing. The period of time during which partial re-breathing occurs and during which normal breathing occurs may be determined by the individual patient""s size and breathing capacity. Additionally, the period of time between a re-breathing episode and a subsequent normal breathing episode may vary between patients, depending on a particular patient""s size and breathing capacity.
Cardiac output or pulmonary capillary blood flow may be determined in accordance with the method of the present invention by estimating the partial pressure of CO2 in the alveoli or the content of the blood in capillaries that surround the alveoli of the lungs of a patient (Ccxe2x80x2CO2), or the alveolar CO2 content (CACO2), rather than basing the cardiac output or pulmonary capillary blood flow determination on the partial pressure of end-tidal CO2, as is typically practiced in the art. Partial pressure values that are obtained from CO2 measurements are converted to a value for gas content in the blood using a CO2 dissociation curve or equation, as known in the art. Thus, a more accurate cardiac output or pulmonary capillary blood flow value can be determined with alveolar CO2 measurements than with partial pressure of end tidal CO2 measurements.
In addition, the accuracy of the cardiac output or pulmonary capillary blood flow measurement may be increased by correcting CO2 elimination values to account for flow of CO2 into the functional residual capacity of the lungs, which is the volume of gas that remains in the lungs at the end of expiration. The cardiac output or pulmonary capillary blood flow of the patient may then be determined by accounting for the functional residual capacity and by employing the values obtained in accordance with the method of the present invention, as well as other determined values, known values, estimated values, or any other values based on experiential data, such as by a computer processor in accordance with the programming thereof. Alternatively, cardiac output or pulmonary capillary blood flow may be estimated without accounting for functional residual capacity.
The ventilation apparatus of the present invention may also employ inexpensive yet accurate monitoring systems as compared to the systems currently used in the art. The methods of the invention may include the automatic adjustment of the deadspace volume of the apparatus to accommodate patients of different sizes or breathing capacities or changes in the ventilation or respiration of a patient, and provides consistent monitoring with modest recovery time. Further, the present apparatus and methods can be used with non-responsive, intubated patients and with non-intubated, responsive patients.